In image forming methods such as electrophotography, in general, a surface of an electrostatic latent image carrier (photoconductor) is charged by corona discharge etc. and then exposed by laser etc. to form an electrostatic latent image, the electrostatic latent image is developed by a toner to form a toner image, and the toner image is further transferred on a recording medium to obtain an image with high quality. The toners used for forming toner images are usually those produced by mixing a binder resin such as thermoplastic resin with a colorant, a charge control agent, a release agent, etc., which are then kneaded, pulverized, classified to form toner particles with an average particle diameter of 5 to 10 μm. Then, in order to provide flowability to the toner, to control a charged amount of the toner, and to improve easiness of cleaning of the toner not transferred and remaining on the photoconductor, inorganic or inorganic metal fine particles such as silica and titanium oxide are externally added to the toner.
Recent years, longer operating life and higher speed are demanded in addition to higher image quality for image forming apparatuses using electrophotography etc. In order to respond to the demand for the image forming apparatuses, an initial rise of triboelectric charging capacity of the toner is important such that the toner can be stably charged to a desired charged amount in a short time. When the initial rise of triboelectric charging capacity of the toner is insufficient, since the development is carried out by the toner of inferior charge, problems such as increase of image density of formed images, generation of image fog in formed images, and toner scattering in image forming apparatuses tend to occur in case using two component developers and problems such as insufficient image density and image non-uniformity tend to occur in case using one component developers.
In order to solve the problems such as the initial rise of triboelectric charging capacity and the charge stability, for example, there is proposed a toner, on the surface of which positively chargeable charge control resin is adhered, by mixing an emulsion, consisting of an aqueous medium containing an organic solvent and a positively chargeable charge control resin, and a suspension of toner mother particles containing a binder resin of a polyester resin and a colorant, and adhering the positively chargeable charge control resin to the surface of the toner mother particles.
The toner described above is excellent in an initial development property since the initial rise of triboelectric charging capacity is excellent and the toner can be charged to a desired charged amount in a short time. However, the positively chargeable charge control resin of the toner described above is no more than one which merely adheres to the toner mother particles. For this reason, when printing with a lower coverage rate is carried out using the toner described above for a long period and thus the toner is stirred in development devices for a long period, the charge control resin tends to drop off from the surface of the toner mother particles. In such a case, since the toner mother particles consist of a polyester resin having negative charging properties, they tend to repel from a negatively charged carrier etc. and smear the image forming apparatuses due to scattering of the toner.